The current trend is to provide louder sound pressure level (“SPL”) at the output of micro-speakers of a variety of devices such as feature phones, smartphones and tablets. Speakers may alternatively be referred to as “loudspeakers” herein. As well known to those of skill in the art, SPL is a measure of the change in sound pressure in front of a speaker caused by its operation.
A problem that occurs when increasing SPL levels in speakerphones is that it often causes the loudspeakers to operate in non-linear regions. When the devices are operated in a speakerphone mode, it becomes difficult to effectively cancel the echo made by direct and reflected acoustical paths between the speaker and the microphone.
FIG. 1 is a block diagram illustrating certain echo canceller systems of the prior art. In a first example, FIG. 1 shows a traditional echo canceller that works by feeding the far-end signal (“speech”) as a reference R1 to an adaptive filter that models the acoustic feedback (“echo”). The output of the adaptive filter is subtracted from the microphone output to reduce the echo in the microphone signal. In a second example, the reference signal R1 is omitted and a reference signal R2 from a sensor S (e.g. an accelerometer or microphone) that is attached to the cone of a speaker is input into the adaptive filter.
A problem with the first example operation of the block diagram of FIG. 1 is that the reference R1 is not an accurate representation or “model” of the actual SPL variation at the speaker, resulting in poor echo cancellation. The second example operation results in a better representation or “model” of the SPL of the loudspeaker, but only at the cost of attaching a sensor S to the cone of the loudspeaker, affecting the speaker's SPL characteristics and adding to the cost of the system.
The error between the modeled acoustic feedback signals (e.g. R1 or R2) and the near-end signal microphone output can be used to adapt the adaptive filter and improve its performance. In a steady state, the adaptive filter learns to precisely model the acoustic feedback and the error is minimized. However, since the reference fed into a traditional echo canceller (e.g. R1) does not carry any information about the non-linearities introduced by loudspeaker, echo cancellers of the prior art that do not use a sensor S cannot cancel non-linear components, leading to more echo residual in the echo cancelled speech.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.